Method and apparatus for determining moisture content of dielectric materials by measuring the ratio of voltage changes at two frequencies



Nov. 3, 1964 G. w. WALLS 3,155,902 METHOD AND APPARATUS FOR DETERMININGMOISTURE CONTENT OF DIELECTRIC MATERIALS BY MEASURING THE RATIO OFVOLTAGE CHANGES AT TWO FREQUENCIES Original Filed'Sept. 5, 1956 2Sheets-Sheet 1 FIG. I

UBTRACTOR AMPLIFIER RECTIFIER FILTER RECTIFIER LOGARITHMIC Nov. 3, 1964G. W. WALLS METHOD AND APPARATUS FOR DETERMINING MOISTURE CONTENT -OFDIELECTRIC MATERIALS BY MEASURING THE RATIO OF VOLTAGE CHANGES AT TWOFREQUENCIES Original Filed Sept. 5, 1956 2 Sheets-Sheet 2 dare/verve PIA1: cl r United States Patent 3,1553% ETHOD AND AEPARATUS FGR DETERWIWWGMOISTURE CONTENT OF DIELECTRIC MATE- REALS BY MEASURENG THE RATIO 0FVOLT- AGE (IHANGES AT TWO FREQUENCIES Gordon William Walls, Geelong,Victoria, Australia, assignor to Industrial Nucieonics Corporation, acorporation of Ohio Continuation of application Ser. No. 663,095, Sept.5, 1956. This application Nov. 16, 1962, er. No. 238,838 11 Claims. (Cl.324-61.)

This invention relates to a method and apparatus for measuring themoisture content of textile fibers and materials and other dielectricmaterials of which the dielectric constant changes as the frequency of avoltage applied to them changes. This is a continuation of copendingapplication Serial No. 608,095, filed September 5, 1956, now abandoned,by Gordon William Walls for vlethod and Apparatus for Determining theMoisture Content of Dielectric Materials.

It is known that the dielectric constant of materials of the above typeis also dependent on the moisture con tent and this moisture content canbe measured by placing or passing the material between the plates of acondenser and measuring the dielectric properties of the material at twodifferent frequencies. It is also known that such measurement can bemade independent of the mass of the material between the plates.

These known methods all suffer from one or more of the followingdefects:

(a) The measurement is not independent of the position of the materialbetween the plates.

(1)) Manual balancing adjustments are required, thus preventing themeasuring instrument from being used to obtain continuous measurement asis desirable, for instance in the measurement of the moisture content ofa moving strip of textile material.

(0) The lack of stability of the measuring instruments is such as toreduce their accuracy to a value considerably below that which istheoretically possible.

(62) They cannot be used effectively on greasy or adulterated materials(e.g., dyed textiles or those containing electrolytes such as sodiumcarbonate).

(e) Measurements are afiected by outside disturbances such as variationin applied voltage and ageing of circuit components.

(f) The measurement is not independent of the temperature of thematerial between the plates.

it is the object of this invention substantially to avoid these abovestated defects.

Throughout the following description and claims, reference will be madeto the use of voltages as the operative electrical quantities in themethods and apparatus according to the invention. It is to beunderstood, however, that other suitable electrical quantities, such ascurrents, could be used; provided that the circuit components were,where necessary, suitably adjusted to make them responsive to thoseother quantities. Accordingly reference hereinafter to voltages is meantto refer also to any othersuch quantity.

Accordingto this invention there is provided apparatus fordeterminingthe moisture content in material of the ypc referred to,comprising a tesL condenser having plates between which the material maybe located, means for feeding to the test condenser at least twovoltages of different frequencies, means for obtaining a resultantvoltage which is a function of the ratio of the change in voltage dropsacross the plates of the test condenser corresponding to each feedfrequency when the material is located between the plates, and meansresponsive to the resultant voltage to give an indication of moisturecontent which is independent of the mass and the position of thematerial between the plates of the test condenser.

The relation between the change in voltage across the plates of acondenser having a material of the type under consideration between theplates can be expressed generally in the form V=ABM, where V is thevoltage change across the plates, A is a constant dependent on theposition of the material between the plates, M is the mass of materialbetween the plates and B is a function of the moisture content of thematerial and of the frequency of the applied voltage. The change involtage is occasioned by the change in admittance between the plates ofthe condenser, the admittance being a function of the mass and moisturecontent. In the case of wool, for instance, it is known that themoisture content can be measured by passing the material be tween theplates of a condenser and measuring the voltage changes corresponding totwo feed frequencies F F applied separately. These voltage changes are afunction of the moisture content and can be expressed with reasonableaccuracy by the relations:

V AB M and V AB M where V; is the voltage change corresponding to onefeed frequency, V is the voltage change corresponding to another feedfrequency and B and B are functions of the moisture contentcorresponding to the frequencies F and F respectively. Dividing theseexpressions we obtain which expresses a relation between voltage changeand moisture content independent of the mass and the position of thetest sample. This realization forms the basis from which the presentinvention has been developed. Thus, in accordance with the principles ofthe invention, the determination of the moisture content depends on themeasurements of the ratio of two simultaneously occurring voltages inwhich any variation other than that due to change in moisture contentoccurs in such manner that any change in one of them will be compensatedfor by a corresponding change in the other so that, within reasonablelimits of accuracy, the ratio remains unchanged.

Thus this invention includes a method for quantitatively determining atleast one property of a dielectric material by measuring a function ofthe dielectric properties of the material, how they vary with frequency.According to this invention the material to be tested is coupled to atest condenser to which voltages of at least two different frequenciesare applied. Then a signal is obtained that is a function of the ratioof changes in voltage drops across the test condenser at the respectivefeed frequencies when the material is coupled to the test condenser. Thechanges in voltage drops are occasioned by changes in the admittance ofthe test condenser as the dielectric is changed.

Accordingly, it is the primary object of this invention to provide amethod and apparatus for determining quantitatively at least oneproperty of a dielectric material by measuring the variation ofdielectric constant with frequency. It is a further object to make sucha determination independent of variations in at least one other variableproperty that affects the admittance of the test condenser. It is astill further object to determine moisture content of dielectricmaterial independent of the mass of the material.

Other objects and advantages will become apparent from the followingdetailed description when considered with the accompanying drawings inwhich:

FIGURE 1 is a schematic diagram of one form of apparatus embodying theinvention;

FIGURE 2 is a schematic diagram of an alternative form of apparatusembodying the invention; and

FIGURE 3 is a circuit diagram showing a particular circuit arrangementwhich may be employed.

More particularly apparatus according to the invention comprises meansfor generating two voltages of different frequency which are fedsimultaneously to means for obtaining two resultant voltages which aredependent on the moisture content of the material, means for filteringand thus separating the two resultant voltages, means for obtainingvoltages which are proportional to the logarithms of the two resultantvoltages and means for subtracting the two logarithmic voltages thusobtained to obtain a logarithmic voltage which is a function of theratio of the two resultant voltages, said last mentioned logarithmicvoltage being capable of being applied to a recording or indicatingmeter.

The apparatus may also include means for rectifying the voltages at somestage after filtering and before subtraction and it may also includeamplifying means at any suitable point or points in the circuit.

This form of the invention is illustrated in FIGURE 1. As shown inFIGURE 1, a pair of oscillators 1, 2 are connected to a capacitancebridge 3 one of the capacitors 4 of which is used as the test capacitorbetween the plates of which the material, the moisture content of whichit is desired to determine, is located. Before the material is placedbetween the plates of the test condenser the bridge is balanced ornearly balanced so that there is substantialy no voltage differencebetween the output terminals of the bridge. The output terminals of thebridge 3 are connected to filters 6, 7 which serve to separate thesignals to produce a resultant voltage at each of the appliedfrequencies. These separate voltages may then be rectified by rectifiers8, 9 and applied to logarithmic amplifiers 5, 5 to produce voltagesproportional to the logarithms of V and V These logarithmic voltages arethen applied to a subtractor 10 in which they are subtracted to give avoltage which is proportional to the logarithm of the ratio of thevoltage V and V This voltage, which is proportional to log V /V isapplied to a suitably calibrated indicating or recording meter 11 so asto give a direct instantaneous or continuous reading of moisturecontent, independent of changes in mass.

In an alternative apparatus according to the invention, the filtering ofthe voltage may occur after the voltages have been converted intologarithmic voltages.

In this form of the apparatus, as shown in FIGURE 2, the oscillators 1,2 are connected as in the apparatus shown in FIGURE 1 and the bridge 3and capacitor 4 are as described above. However, in this form of theinvention, the output terminals of the bridge 3 are connected directlyto a logarithmic amplifier 5 by means of which the logarithms of the twovoltage changes, V V obtained from the bridge 3 when the material islocated between the plates of the test condenser 4 are obtained andthese voltages are then fed to filters 6, 7, the voltage passed by thefilter 6 being proportional to the logarithm of the voltage V and thevoltage pased by the filter 7 being proportional to the logarithm of thevoltage V These two logarithms voltages are then rectified by rectifiers8, 9 and applied to a subtractor 10 in which they are subtracted to givea voltage which is proportional to the logarithm of the ratio of thevoltages V and V This resultant voltage which is proportional to log V/V is applied to a suitably calibrated indicating or recording meter 11,so as to give a direct, either instantaneous or continuous, reading ofmoisture content.

FIGURE 3 shows in more detail one particular circuit which has beendesigned for carrying the invention into practical effect and whichcorresponds to the apparatus described with reference to the schematicdiagram of FIGURE 2.

Tubes 12 and 13, together with their associated components, for a Wienbridge oscillator corresponding to the oscillator 1 of FIGURE 2 andoscillating at a frequency F which for practical purposes may be chosenat 10 cycles/ second, while tubes 14 and 15 together with theirassociated components, for a second Wien bridge oscillator correspondingto oscillator 2 of FIGURE 2 and oscillating at a frequency F which maybe 50X 10 cycles per second. It should be noted that the frequenciesemployed are not critical and need not correspond to the particularfrequencies stated. All that is necessary is that they are:

(a) Far enough apart so that they may later be separated after mixing,

(b High enough to allow the use of standard circuit components (this ofcourse is a purely economic consideration to avoid the necessity foremploying specially made chokes etc.).

In operation, the two frequencies from the oscillators are fed to thebridge 3, the capacitor 4 of which forms the test capacitor of theapparatus. The material to be csted is placed between the plates of thistest capacitor and the resultant out-of-balance voltages of the bridgeare fed to an amplifier tube 16 and thence to tubes 1'7 and 18 whichaltogether form the logarithmic amplifier 5 of a known type. The twofrequencies are not maintained separately in this amplifier but areallowed to mix therein. Under some circumstances this is of advantagesince any changes in the properties of the tubes, particularly thoseafiecting amplification, will affect both frequencies to the same extentand will therefore tend to cancel out. The output from the tube 18 ofthe logarithmic amplifier 5 is then fed through a transformer to tunedcircuits I9 and 26 which form the filters 6 and 7, respectively, toseparate the two frequencies F and F so that the voltage input to a tube21 is substantially of frequency F and the input voltage to a tube 22 issubstantially of frequency F The tubes 21 and 22 act as amplifiers and,since the voltages fed to them have previously been logarithmicallyamplified and then separated by tuned circuits 19 and 20, the outputfrom tube 21 will be pro portional to the logarithm of theout-of-balance voltage across the bridge at frequency F Further tunedcircuits 23 and 24 on the output sides of tubes 21 and 22 serve tofurther filter the outputs and ensure that the frequencies are properlyseparated. These separated outputs are then fed to separate parts of arectifier tube 25 which correspond to the rectifiers 8 and 9 of FIGURE 2and which provide direct current voltages proportional to the voltagesof the two frequencies fed to them. These direct current voltages cannow be subtracted and this is achieved by applying them to thesubtractor It) so that the voltage between the points A and B isproportional to the difference between the two voltages. Thus thevoltage between A and B is proportional to log V /V where V is thebridge-out-of-balance voltage at frequency F and V is thebridge-out-of-balance voltage at frequency F This voltage issubstantially independent of the mass of wool between the plates of thetest condenser 4 and of its distribution between the plates sincechanges of mass or distribution would affect the voltages at eachfrequency similarly and the resultant voltage would remain unchanged.

This resultant voltage, which is proportional to log V V is then appliedto the indicating or recording meter 11 to indicate variations inmoisture content. Preferably the meter is of the type having a zerocenter and is also supplied with a direct current source 26 whichprovides a voltage between the points B and C which is proportional tolog V /V where V and V are the bridge-out-of balance voltages atfrequency F and F at the desired moisture content. The meter is soarranged that when log V /V equals log V /V' the meter indicates Zero.If the material is too Wet the meter will deflect in one direction andif it is too dry it will deflect in the opposite direction in accordancewith whether V V is greater or less than V /V' A condenser 27 and aresistance 23 may also be placed in the meter circuit to ensure that 5short term fluctuations in moisture content will not affect the meter.

In a modification of the apparatus which is not shown in the drawings,the capacitance bridge 3 is replaced by a test condenser which isarranged in series with a standard condenser having a capacitance whichis large relative to the test condenser and is simultaneously suppliedwith AC. at two different frequencies. The voltage changes across thestandard condenser corresponding to the two frequencies when material ispassed between the plates may be represented by V and V These voltages Vand V are then separately amplified and rectified bysuitable means suchas a pair of amplifying pentodes, the grid circuit of one being tuned tothe higher frequency and the grid circuit of the other being tuned tothe lower fr q'uency, followed by two diode detector valves. Thevoltages are then fed, for instance, into a pair of logarithmicamplifiers to give, after suitable subtraction, a voltage which isproportional to log V V We have, therefore, a voltage which is afunctionof the ratio V /V which, as previously shown, is equal to B /B If asingle source, for instance a pentode valve, is used to supply the twofrequencies and only one valve is used to logarithmically amplify thetwo measured voltages before rectification, the added advantage of thecancellation of any outside disturbances is obtained since any change inone voltage will be compensated for by a corresponding change in theother voltage so that an ultimate null ei'fect is produced.

The fact that the moisture content 'is indicated by a function of theratio 'V /V and the two voltages are measured simultaneously allows themoisture content of adulterated material such as greasy wool or dyedtextiles to be measured since each of the voltages V and V will besimilarly affected and there will be no substantial change in the ratiodue tothese effects.

Although the above description has been directed in pmticular, to theuse of the invention with wool and other textile materials, it isapplicable to other dielectric materials such as tobacco, of which thedielectric constant changes as the frequency of a voltage applied tothem changes. The circuit elements particularly described may also bevaried Without departing from the spirit and scope of the invention.

What is claimed is:

1. Apparatus for determining the moisture content in material of thetype referred to, comprising a test condenser having plates betweenwhich the material may be located, means for coupling to the testcondenser at least two voltages of different frequencies, means coupledto the test condenser for obtaining a resultant voltage indicative ofthe ratio of the change in magnitude of voltage drops across the platesof the test condenser corresponding to each frequency when the materialis located between the plates, and means responsive to said resultantvoltage to give an indication of moisture content which is independentof the mass and the position of the material between the plates of thetest condenser.

2. A system for quantitative measurement of at least one property of adielectric material having a plurality of variable properties eachaffecting the admittance between electrodes coupled by said material,said system comprising capacitive measurement probe means having spacedelectrodes for coupling said probe means to said material, a pluralityof voltage sources of different frequencies, means coupled to saidsources for electrically energizing said electrodes from said sources ofdifferent frequencies, means for detecting the signals developed betweensaid electrodes at each of said frequencies by producing detectionsignals indicative thereof, means coupled to said last named means forbalancing out from said detection signals at each of said frequenciesany component developed in the absence of said material at said probemeans, and means for combining said detection signals to produce acontinuous output measurement rep- 6 resentative of said one propertyand substantially independent of variations in at least one othervariable property of said material.

3. Apparatus for determining the moisture content of material under testcomprising capacitive probe means having spaced electrodes for couplingsaid probe means to said material, means for energizing said capacitiverobe means at at least two different frequencies, and means responsiveto signals developed across said capacitive probe means induced by saidmaterial at said different frequencies for producing a continuous outputmeasurement representative of moisture content which is independent ofvariations in the mass of the material under measurement, said meansresponsive to signals including means for balancing out from saidsignals at each of said frequencies any component developed in theabsence of said material at said probe means.

4. Apparatus for determining the moisture content of material under testcomprising a capacitive probe means having spaced electrodes forcoupling said probe means to said material, means for energizing saidcapacitive probe means at at least two different frequencies, meansresponsive to signal increments across said capacitive probe meansinduced by said material at said different frequencies for producing acontinuous output signal proportional to the ratio of said signalincrements, and

means responsive to said output signal for producing a measurementrepresenting moisture content which is independent of variations in themass of the material under measurement.

5. Apparatus for quantitative determination of at least one p'ropertyofa dielectric material, said apparatus comprising a plurality of sourcesof electrical signals at different respective frequencies, detectingmeans, and capacitive probe means coupled to said sources and havingspaced electrodes arranged for applying'elect'rical signals from saidsources to at least a portion of said material and coupling saiddetecting means to said portion, said detecting means including meansfor deriving separate signals each -resulting from the signals appliedto said portion from a respestive source as these applied signals areinfluenced by the mass of said material and the dielectric properties ofsaid material at the frequency of that source, and means for combiningsaid separate signals to produce a continuous resultant signal that is afunction of the ratio of said separate 'signals and quantitativelyindicative of said property.

6. Apparatus for quantitative deter'mination'of at least one property ofa dielectric material, said apparatus comprising a plurality of sourcesof electrical signals at different respective frequencies, detectingmeans, and capacitive probe means coupled to said sources and havingspaced electrodes arranged for applying electrical signals from saidsources to at least a portion of said material and coupling saiddetecting means including means for deriving separate signals eachresulting from the signals applied to said portion from a respestivesource as these applied signals are influenced by the mass of saidmaterial and the dielectric properties of said material at the frequencyof that source, and means for combining said separate signals to producea continuous resultant signal that is quantitatively indicative of saidproperty, said apparatus further including a capacitor connected in circuit with said probe means to balance out from said separate signals anysignals developed in the absence of said material at said probe means,whereby said separate signals include substantially only the changeoccasioned by the introduction of said dielectric material to said probemeans.

7. Apparatus for quantitative determination of at least one property ofa dielectric material, said apparatus comprising a plurality of sourcesof electrical signals at different respective frequencies, detectingmeans, and capacitive probe means coupled to said sources and havingspaced electrodes arranged for applying electrical signals from a saidsources to at least a portion of said material and coupling saiddetecting means to said portion, said detecting means including meansfor deriving separate signals each resulting from the signals applied tosaid portion from a respective source as these applied signals areinfluenced by the mass of said material and the dielectric properties ofsaid material at the frequency of that source, and means for combiningsaid separate signals to produce a continuous resultant signal that is afunction of the ratio of said separate signals and quantitativelyindicative of said property, said apparatus further including a bridgecircuit coupling said sources to said capacitive probe means, saidcapacitive probe means being connected in one arm of said bridgecircuit, said bridge circuit including a capacitor connected in anotherarm thereof, said capacitor providing such capacitance as to balancesaid bridge circuit in the absence of said material at said probe means,whereby said separate signals include substantially only the changeoccasioned by the introduction of said dielectric material to said probemeans.

8. A method for quantitatively determining at least one property of adielectric material having a plurality of variable properties eachaffecting the admittance between electrodes coupled by said material,said method comprising applying electrical signals at a plurality ofdifferent frequencies to at least a portion of the material under test,deriving separate output signals each indicative of the differencebetween the signal thereupon developed between electrodes coupledtogether by a portion of said material to which said applied signals areapplied at a respective frequency and the signal developed between saidelectrodes at said respective frequency in the absence of said material,and combining said separate signals to produce a continuous resultantsignal that is quantitatively indicative of said property andsubstantially independent of variations in at least one other variableproperty of said material.

9. A method for the quantitative determination of at least one propertyof a dielectric material, said method comprising applying electricalsignals at a plurality of different frequencies to at least a portion ofthe material, deriving separate signals each resulting from the signalsapplied at a respective frequency as these applied signals areinfluenced by the mass of said material and the dielectric properties ofsaid material at that frequency, and combining said separate signals toproduce a continuous resultant signal that is a function of the ratio ofsaid separate signals and quantitatively indicative of said property.

10. A method for quantitatively determining at least one property of adielectric material, said method comprising passing said materialcontinuously through a test region, applying electrical signals at aplurality of different frequencies to the material in said region,deriving separate output signals each indicative of the differencebetween the voltage developed between electrodes coupled together bysaid material in said region at a respective frequency and the voltagedeveloped between said electrodes in the absence of said material atsaid respective frequency, and combining said separate signals toproduce a continuous resultant signal that is a function of the ratio ofsaid separate signals and quantitatively indicative of said property.

11. The method of quantitatively determining the moisture content of adielectric material under test which comprises applying electricalsignals at first and second significantly difierent frequencies to apair of spaced electrodes defining a field region, continuously passingsaid material through said field region whereby the admittance of saidelectrode pair varies as a function of the dielectric properties of saidmaterial in said field region, generating a first signal indicative ofthe difference between said admittance at said first frequency and theadmittance of said electrode pair at said first frequency in the absenceof said material, generating a second signal indicative of thedifference between said admittance at said second frequency and theadmittance of said electrode pair at said second frequency in theabsence of said material, and continuously combining said signals toproduce a continuous resultant signal that is a function of the ratio ofsaid first and second signals and quantitatively indicative of saidmoisture content.

References Cited in the tile of this patent UNITED STATES PATENTS2,527,208 Berry et al. Oct. 24, 1950 2,535,026 Anderson Dec. 26, 19502,535,027 Anderson Dec. 26, 1950 2,576,249 Barney Nov. 27, 19512,625,583 Broding Jan. 13, 1953 2,744,233 Paivinen May 1, 1956 2,908,166Johnson Oct. 13, 1959 2,920,206 Heller Jan. 5, 1960 2,929,020 Mayes Mar.15, 1960 2,950,435 Locher et al. Aug. 23, 1960 FOREIGN PATENTS 622,470Great Britain May 3, 1949

9. A METHOD FOR THE QUANTITATIVE DETERMINATION OF AT LEAST ONE PROPERTYOF A DIELECTRIC MATERIAL, SAID METHOD COMPRISING APPLYING ELECTRICALSIGNALS AT A PLURALITY OF DIFFERENT FREQUENCIES TO AT LEAST A PORTION OFTHE MATERIAL, DERIVING SEPARATE SIGNALS EACH RESULTING FROM THE SIGNALSAPPLIED AT A RESPECTIVE FREQUENCY AS THESE APPLIED SIGNALS AREINFLUENCED BY THE MASS OF SAID MATERIAL AND THE DIELECTRIC PROPERTIES OFSAID MATERIAL AT THAT FREQUENCY, AND COMBINING SAID SEPARATE SIGNALS TOPRODUCE A CONTINUOUS RESULTANT SIGNAL THAT IS A FUNCTION OF THE RATIO OFSAID SEPARATE SIGNALS AND QUANTITATIVELY INDICATIVE OF SAID PROPERTY.